allow converting from DateTime2 to Datetimen in 'tds73'

src/tds/codec/column_data.rs

@@ -25,6 +25,7 @@ mod xml;
 use super::{Encode, FixedLenType, TypeInfo, VarLenType};
 #[cfg(feature = "tds73")]
 use crate::tds::time::{Date, DateTime2, DateTimeOffset, Time};
+use crate::time::chrono::from_days;
 use crate::{
     tds::{time::DateTime, time::SmallDateTime, xml::XmlData, Numeric},
     SqlReadBytes,
@@ -594,6 +595,22 @@ impl<'a> Encode<BytesMutWithTypeInfo<'a>> for ColumnData<'a> {
                 time.encode(&mut *dst)?;
             }
             #[cfg(feature = "tds73")]
+            (ColumnData::DateTime2(opt), Some(TypeInfo::VarLenSized(vlc)))
+                if vlc.r#type() == VarLenType::Datetimen =>
+            {
+                if let Some(dt2) = opt {
+                    dst.put_u8(8);
+                    let days = dt2.date().days().checked_sub(693595).ok_or_else(|| crate::Error::Conversion(
+                    format!("invalid datetime, expecting datetime not earlier than 1900-01-01 but got {:?}",
+                            from_days(dt2.date().days() as i64,1)).into()))?; // minus days gap between year 1 and year 1900
+                    let seconds_fragments = dt2.time().increments() * 100 * 300 / 1e9 as u64; // degrade second's precision
+                    dst.put_u32_le(days as u32);
+                    dst.put_u32_le(seconds_fragments as u32);
+                } else {
+                    dst.put_u8(0);
+                }
+            }
+            #[cfg(feature = "tds73")]
             (ColumnData::DateTime2(opt), Some(TypeInfo::VarLenSized(vlc)))
                 if vlc.r#type() == VarLenType::Datetime2 =>
             {

src/tds/time/chrono.rs

@@ -18,7 +18,7 @@ pub use chrono::{DateTime, NaiveDate, NaiveDateTime, NaiveTime};
 use std::ops::Sub;
 
 #[inline]
-fn from_days(days: i64, start_year: i32) -> NaiveDate {
+pub fn from_days(days: i64, start_year: i32) -> NaiveDate {
     NaiveDate::from_ymd_opt(start_year, 1, 1).unwrap() + chrono::Duration::days(days)
 }